This invention relates generally to laminates for printed circuits and more specifically to the woven glass fabric used as a reinforcing filler in such laminates.
Cloth woven of warp and fill yarns composed of fine, twisted glass filaments, also known as unplied yarns, is widely used as reinforcement and substrate filler material for printed circuits. The cloth is typically impregnated in a treater with a liquid resin of organic polymer such as epoxy, then partially polymerized or semi-cured by heat to an easily handled material known as prepreg. It is laminated with other prepreg sheets or metal foil to full polymerization or final cure under heat and pressure. The foil is etched to form circuits and multiple sets of laminate can be joined. Thereafter, holes are drilled for circuit vias from one surface of the laminate to the other to enable connections between circuit lines on the opposite panel surfaces or intermediate circuit planes in multilayer panels.
The via or through hole walls are plated with conductive material that is usually copper to provide electrical continuity. For the plating process, the newly-drilled holes are cleaned with a liquid, seeded with a plating accelerator in a bath, then metal-plated by immersion in an electroless plating solution After these circuitized substrates have been in service varying lengths of time, failures frequently have occurred as short circuits.
Analysis of these failures has revealed that metal, usually copper, has plated onto the interior of the glass yarns along the filament surfaces extending from the hole walls, eventually contacting a nearby voltage or ground plane or another hole wall. The spontaneously plated metal has resulted from plating solution being trapped in voids along the glass filaments of the warp yarn and subjected to applied voltage differentials. The solution becomes trapped in the voids from the time the hole walls were initially immersed in plating solution. After plating had been completed, the plating solution had been unable to drain from the filament voids. The voids generally were elongated and occurred near the centers of the warp yarns. This indicates that the glass yarns were not wholly or entirely impregnated with resin so that voids existed among the bunched filaments forming those yarns. Such voids occurred only infrequently within the orthogonal fill or weft yarns.
A subsequent finishing step for the cloth in the manufacturing process is that of removing filament lubricant coatings. The fabric is subjected to high temperature that has the ancillary effect of further tending to fix the cross sectional dimensions of the yarn and prevent later yarn relaxation or expansion.
The difficulty of thoroughly impregnating the unplied glass yarns has been discussed in a paper by G. Wiedemann, H. Rothe, A. Hasse and R. Wiechmann entitled "Relations Between Properties of Wetting and Impregnation, Technology, and Application of FRP", Advanced Composition of Matter, Proceedings International Conference, 3rd, Volume 2, pages 1636-51, 1980. On pages 1641 and 1642, the authors describe the presence of capillary lines or hollow spaces among filaments, particularly in the warp yarns. Their investigation concluded that the total impregnation of yarns was hindered by gumming due to the coupling agents used for adherence and to the density of the packed filaments in the yarn requiring greater displacement of air.
One of the proposed solutions to the problem of internal voids within twisted, unplied glass yarns has been to use plied yarns for both warp and fill. Plied yarns are composed of two or more twisted strands wrapped about each other in the fashion of a rope. Each strand has a plurality of glass fibers or filaments. The strands are twisted a predetermined number of times per unit length as with the usual unplied yarns and then wrapped in rope fashion. Advantageously, the wrapping is done in a direction that, in essence, untwists the pretwisted yarns so that the bundles of parallel filaments in each strand overlie each other. This in effect loosened the bundles of filaments to allow elliptical cross sections with shorter conjugate axes allowing easier resin impregnation of the cloth.
The use of plied yarns as filler and reinforcement in printed circuit substrates resulted in more thorough and satisfactory resin impregnation by the organic polymers and radically reduced the problem of yarn voids. Plied yarn substrates, however, have the inherent difficulty of dimensional instability. Although the use of plied yarns was acceptable for substrates having only a few prepreg layers in the composite circuit panel, the substitution of plied yarn prepreg brought unacceptable shifting and dimensional change as the number of prepreg layers were increased to provide a larger number of circuit layers in the ultimate, multilayer composite panel or board.
The wrapping of two or more strands results in the regular crossing of the filament bundles so that the cloth thickness is slightly greater when the regular or full number of filaments is used in each yarn. During impregnation, pressing and semi-curing at the prepreg stage, stresses are induced. Subsequent laminating and final curing under heat and pressure then caused dimensional changes of the circuit panel in which panel length or width vary irregularly. This produces an unacceptable shifting and the potential for short circuits among the layers of the ultimate product.
In an attempt to improve dimensional stability, the size of the plied yarns was decreased by reducing the number of glass filaments in each yarn. The effect of this is to reduce the crimp angle at the intersection where each yarn crosses an orthogonal yarn. As a consequence, the dimensional stability of the prepreg improves but more layers of prepreg are necessary to establish the required dielectric constant between circuit layers. Additional layers translate directly into greater expense. There are required the use of more expensive yarn since it is plied, a greater quantity of woven cloth for more layers, increased resin treatment for more cloth and longer processing times. The attainment of required dielectric constants meant increased costs.